The present invention relates to an image forming apparatus for transcribing an image onto a recording sheet by using toner. Particularly, the present invention is directed to a suction-type sheet carrying mechanism for a high speed image forming apparatus.
Recently, an image forming apparatus has been widely used for transcribing an image onto a recording sheet by using toner as seen in an electrophotographic duplicator. The image forming apparatus is separated into two types, a low speed type operating at a low transcribing speed such as 10 sheets per minute and a high speed type operating at a high transcribing speed such as 100 sheets per minute. The present invention relates to a high speed type image forming apparatus.
In the image forming apparatus, a train of electrical image signals to be transcribed is fed to the image forming apparatus, then a toner image is developed on a photosensitive surface of an image forming medium such as a rotated photosensitive drum or circulated belt, the toner image is transcribed onto a recording sheet at an image transcription unit, and then a toner image transcribed on the recording sheet is fixed at a fixing unit.
In the image transcription unit, the toner image on the image forming medium is transcribed onto the recording sheet by a charging action. However, the charge is discharged also in the image transcription unit after the image transcription on the recording sheet is over. Therefore, the charge still remains on the recording sheet, so that the recording sheet is hard to separate from the photosensitive surface.
The recording sheets fed to the image transcription unit are sent from a sheet cassette and those passed through the image transcription unit are sent to a sheet stacker through a fixing unit. Such a transfer of the recording sheet is performed by a sheet carrying mechanism. The sheet carrying mechanism is divided into two: a first sheet carrying mechanism for carrying the sheet from the sheet cassette to the image transcription unit and a second sheet carrying mechanism for carrying the sheet from the the image transcription unit to the sheet stacker. The present invention is directed to the second sheet carrying mechanism.
In the high speed image forming apparatus, a metal belt, forming a photosensitive layer thereon, has been used as the image forming medium. However, a photosensitive drum, which will be simply called a "drum" hereinafter, is better than the metal belt because the high speed image transcription can be performed more economically; is more stable, and has a longer life. However, a diameter of the drum cannot be made small because the units for toner image forming, developing and transcription are all required to be arranged close to the drum. Therefore, there is a problem in the second carrying mechanism that the recording sheet just passed through the image transcription unit tends to be stuck electrostatically to a cylindrical surface of the drum. In other words, the recording sheet just passed through the image transcription unit is difficult to separate from the cylindrical surface when the usual belts and rollers or pulleys are used in the second carrying mechanism. As long as the usual belts and rollers are used in the second carrying mechanism, the usual belt type image forming medium may be superior, compared with a drum type image forming medium because the belt type medium can be turned with a small radius, using a roller having a small radius, at an ejecting point of the recording sheet from the image transcription unit. This can provide a potential problem in the second sheet carrying mechanism.
In the second sheet carrying mechanism of the high speed image forming apparatus using the drum, there are still two other problems with respect to a very strong fixing flush light irradiated from the fixing unit. The very strong flush light is required to fix the toner image onto the recording sheet being carried at high speed.
One of two other problems is that a sheet carrying route between the image transcription unit and the fixing unit is required to be curved and run through a narrow portion for preventing the fixing flush light from leaking onto the cylindrical surface of the drum along the sheet carrying route. Therefore, the recording sheet cannot be reliably carried by the usual sheet carrying mechanism. Particularly, since a surface of the recording sheet on which the toner image is transcribed is very delicate before the toner image is fixed, the recording sheet must be tightly set on the sheet carrying mechanism and carefully carried. This is not easy at the high carrying speed of the recording sheet.
The other problem is that a nonmetallic belt cannot be used for carrying the recording sheet through a fixing path in the fixing unit because the nonmetallic belt is damaged by heat due to the flush light. However, a metallic belt is difficult to use for high speed revolution and has a short.
These problems can be solved by introducing a suction-type sheet carrying mechanism in the second sheet carrying mechanism. The suction-type sheet carrying mechanism is a mechanism by which the recording sheet is stuck to a carrying medium such as a belt by air suction.
The suction-type sheet carrying mechanism has been applied to the sheet carrying mechanism. The Japanese laid open patent application SHOH 58-55957 to J. SHINOZAKI on Apr. 2, 1983, is an example of a device for separating the recording sheet from drum surface. According to SHINOZAKI, the sheet is separated from the drum surface by a separation unit applying the air suction technique located at the sheet exit of the image transcription unit. The separation unit comprises an endless belt having a plurality of belt holes and an air suction mechanism associated with the belt holes. The air suction at the air suction mechanism is carried out, particularly by applying an air valve in an air exhausting route for increasing an instantaneous suction force of air. The feature of SHINOZAKI is to perform air suction using belt holes. Accordingly, this reference assumes there is a lot of air leakage at the connection between the belt holes and the air suction mechanism because the structure of the air suction mechanism is too complicated, which may be a reason of introducing the air valve in the air exhausting route.